Rotor blade shroud and vibration damping structure



Aug. 7, 1962 -r. FOSTER ET AL.

ROTOR BLADE SHROUD AND VIBRATION DAMPING STRUCTURE Filed Dec. 22, 1959 3Sheets-Sheet 1 Ila ATTORNEY 1962 T. FOSTER ET AL 3,048,365

ROTOR BLADE SHROUD AND VIBRATION DAMPING STRUCTURE Filed Dec. 22, 1959 3Sheets-Sheet 2 INVENTORS TDM FEIETER RICHARD F. .JAKLlTSEH HENRY B. MAYRCHAPMAN \J.WALKER ATTORNEY 22 WNW v I 34 y K 26 Aug. 7, 1962 T. FOSTERET AL ROTOR BLADE SHROUD AND VIBRATION DAMPING STRUCTURE Filed Dec. 22,1959 3 Sheets-Sheet 3 INVENTOR8 TIJM FEIETER RICHARD F. JAKLITSEH HENRYE. MAYH CHAPMAN I.WALKE.R

United States Patent OfiiCfi r 3,048,365 Patented Aug. 7, 1962 3,048,365ROTOR BLADE SHROUD AND VIBRATION DAMPING STRUCTURE Tom Foster,Ridgewood, N.J., Richard F. Jaklitsch, Maspeth, N.Y., and Henry B. Mayr,Ridgefield Park, and Chapman J. Walker, Saddle River, N.J., assignors toCurtiss-Wright Corporation, a corporation of Delaware Filed Dec. 22,1959, Ser. No. 861,311 Claims. (Cl. 253-77) This invention relates torotary fluid compressors, turbines or like apparatus, such as foraircraft jet engines, and having a plurality of circumferentially-spacedblades extending outwardly from their rotors and is particularlydirected to a rotor blade shroud structure having means for dampingblade vibrations.

Although, .as will be apparent, the invention is generally applicable tocompressors, turbines and like apparatus, for convenience it will bedescribed in connection with compressor apparatus.

The weight of fluid compressors can be materially decreased by usinglonger and thinner blades and by designing the compressor for operationat higher speeds. Any such weight reduction obviously is quite importantin the case of air compressors for aircraft engines, as for exampleturbojet engines. Such long and thin high speed compressor blades,however, introduce serious blade vibration problems in bending, bothfundamental and secondary modes, and in torsion Whereas with the morerigid blades of the prior art only bending vibrations in the fundamentalmode were important. These vibration problems are particularly seriousfor the longer blades of the initial rotor stages of a multi-stage axialflow compressor as compared to the shorter rotor blades of the moredownstream stages.

It has been found that if the outer ends of such long and thin highspeed compressor rotor blades are rigidly connected together by a shroudextending across the blade tip ends the stresses in the blades adjacentthe shroud become excessive. In fact, in a particular turbojet enginecompressor application, with such a rigid shroud connection across theblade tips, the stresses in the blades adjacent to the shroud were foundto be as much as three times the blade stresses at their root ends.

An object of the present invention resides in the provision of a noveland simple structure connected across the tip ends of the rotor bladesso as to permit relative motion of the blade tip shroud and to dampblade vibrations both in bending and torsion.

If the shroud for the rotor blades is made sufliciently strong tosupport itself against the centrifugal forces thereon then it will betoo stiff to distribute these centrifugal forces uniformly to eachblade. A further object of the invention resides in the provision of anovel flexible shroud construction such that the shroud can bendslightly under the centrifugal forces to distribute said forcessubstantially uniformly to the rotor blades.

In accordance with the invention the shroud is carried by and isdisposed across the outer ends of the rotor blades with each bladehaving a radially inwardly directed surface engaging a surface on theshroud to restrain the shroud against radially outward movement, saidengaging surfaces being arranged for relative frictional movement todamp blade vibrations. More specifically, the shroud consists of aplurality of flexible sheet metal elements circumferentially disposed inendto-end relation with each shroud element having a plurality ofcircumferentially-spaced and relatively rigid portions, one for eachblade and providing the aforementioned surface engaged by its blade witheach pair of said engaging surfaces having a loose-fitting ball andsocket configuration.

Other objects of the invention will become apparent upon reading theannexed detail description in connection with the drawing in which:

FIG. 1 is an axial sectional view of a compressor rotor stage embodyingthe invention;

FIG. 2 is a developed plan view of several of the shroud elementsdisposed in end-to-end relation;

FIG. 3 is an enlarged developed plan view taken along line 33 of FIG. 1;

FIGS. 4 and 5 are sectional views taken along lines 44 and 5-5respectively of FIG. 3; and

FIG. 6 is an exploded isometric view of a blade shroud, the adjacent endof a blade and the means interconnecting said shroud and blade.

Referring now to FIGS. 2-6 along with FIG. 1, an axial flow compressoris generally indicated at 10 and includes a rotor disc 12 having an axis14. A plurality of circumferentially-spaced blades 16 extend radiallyoutwardly from and are secured to the periphery of the rotor disc, forexample, by a conventional fir tree type connection, at 18. A key 20 isprovided for locking the blades against axial displacement from therotor disc.

Referring now to FIGS. '2-6 along with FIG. 1, an annular shroudconstruction is carried by the rotor blades across their outer or tipends. Said shroud construction comprises a plurality of elongate andflexible shroud elements 22 disposed in end-to-end relation across thetip or outer ends of the rotor blades to form a continuous annularshroud for said blades. Each such shroud element 22 has a plurality ofcircumferentially-spaced pairs of openings 26 with the two openings ofeach pair being for a single blade and lying along a line parallel tothe chordwise direction of the adjacent blade tip. Each blade 16 has twochordwise-spaced tenon portions 28 extending radially-outwardly from itsouter or tip end and projecting through a pair of openings 26 in itsassociated shroud element.

Each shroud element 22 is of thin sheet metal construction and has anoutwardly turned flange 29 running along its twocircumferentially-extending edges and its two end edges. The two endedges of adjacent shroud elements abut along a line 31 FIGS. 2 and 3which may, as illustrated, be parallel to the chordwise direction of theadjacent blade tips. An extremely light weight construction is therebyprovided for the shroud elements 22 whereby said elements can bend inresponse to the centrifugal forces thereon during rotor rotation.

The annular shroud construction also includes a plurality ofcircumferentially-spaced cradle elements 30 which are disposed along theradially-outer side of shroud element 22. There is one such cradleelement 30 for each blade 16 and each cradle element 30 has a relativelyrigid construction compared to that of the shroud elements 22. Eachcradle element 30 has a pair of openings 32 which are alined with a pairof openings 26 in the shroud element so that the tenons 28 of a bladealso project through the cradle openings 32. Hence, the cradle elements30' can be assembled in position on their respective shroud elements 22after each shroud element has been placed in position over its blades16. Each shroud element 22 has outwardly-turned flanges 34 formed aboutits openings 26 thereby helping to locate the cradle elements 30 inposition on the shroud elements.

A bridge element 36 is connected across the ends of the projecting bladetenons 28 as by rivets 38. Each bridge element 36 has aradially-inwardly projecting portion or pin '40 having a semi-sphericalend which extends into a recess or socket 42 formed in a relativelyheavy web 44 of the associated cradle 30, said web 44 separatmg theholes 32 in said cradle As is apparent from the drawing, the axis of thepin and socket connection 40, 42 between each blade 16 and a shroudelement 22 substantially coincides with the longitudinal axis of theblade. The bottom of each cradle socket 42 is also approximatelysemi-spherical but of somewhat larger radius than that of the end of thepin 40. Thus the pin 40 and socket 42 in effect provide a loose ball andsocket fit therebetween to permit sliding motion of the ball end of thepin 40 in its socket 42.

During rotation of the rotor each shroud element 22 and its cradleelements are urged radially outwardly against the blade pins 40 by thecentrifugal forces on said elements. In this way the shroud elements 22are floatingly held in position by the centrifugal forces there onurging said shroud elements outwardly against the blade pins 40.Vibration of the blade 16 in torsion results in frictional slidingmotion between the blade pin 40 and the bottom of the shroud cradlesocket 42 engaged by the pin. Bending vibration of a blade resulting inchanges in the angle between the blade tip and associated shroud element22, such as when a node exists at the blade tip, results in angularmotion of the axis of its pin 40 relative to the associated socket 42 toproduce frictional sliding between said pin and socket. This frictionalsliding between the small diameter pin 40 of each blade and its shroudsocket 42 has been found to be sufficient to damp such torsional andbending blade vibrations. Only a small frictional effort is necessaryfor such damping. For example, in a particular turbojet engine havingfirst stage compressor blades with a length of approximately eleveninches suflicient frictional damping was obtained with each pin 40 andsocket 42 having a contact area of only about 0.09 inch radius underload. This small contact area results from deflection of the engagingsocket and pin surfaces under load and is obtained by providing thesocket 42 with a somewhat larger radius than that of the spherical endof the pin 40. If the frictional area of contact or moment arm of thefrictional forces is large then the frictionally contacting blade andshroud surfaces will substantially lock together and result in excessiveblade vibration stresses. Bending vibrations of a blade 16 producinglateral motion of the blade tip also results in limited sliding betweenthe blade pin 40 and shroud socket 42 and in addition such lateralvibratory motion of a blade tip is further damped as a result of all theblade tip ends extending through a single shroud element 22 in effectbeing tied together against substantial relative lateral motions by theball and socket connection 40-42 between said blades and the shroudelement.

As is conventional in such rotor blades each blade chord at its tip endis twisted relative to the blade chord at its root end. Because of theflexibility of each blade the magnitude of this twist changes somewhatwith changes in the centrifugal force on the blade. The holes 26 and 32through which the tenons 28 of each blade 16 extend are of such size asto provide clearance between said tenons and the walls of said holes inall positions of twist of the blade. It should also be noted that theblade tip ends stretch radially and therefore increase theircircumferential spacing in response to the centrifugal forces thereon.However, with the shroud made up of a plurality of individual end-to-endelements 22 which merely abut each other no circumferential stresses aredeveloped in the shroud elements 22 and therefore the circumferentialspacing of the shroud element holes 26 does not increase in response tothe centrifugal forces. Therefore, the holes 26 in the shroud elementsand the holes 32 in the cradles 30 must also be of suflicient size toaccommodate the increased spacing of the blade outer ends resulting fromblade stretching. This factor together with the problem of assembly ofthe shroud elements 22 over the blades limits the lengths of the shroudelements. As illustrated, each shroud element has a length such that itextends over six blades 16. -In an actual compressor design this shroudlength was such as to subtend an angle of about 40 from the rotor axis.Obviously, however, the invention is not limited to this precise numberof blades per shroud element or to this arcuate length of each shroudelement. The shroud elements 22 should not be so long, however, that theaforementioned blade stretching requires excessively large clearancebetween the blade tenons 28 and the walls of the holes 26 nor should itbe so long as to make assembly difl'lcult. However, each shroud elementpreferably should be long enough to extend over a sufficient number ofblades 16 such that it is not likely that all these blades will havesimilar lateral vibrations of their blade tip ends at any one time.

The recess 42 in each shroud cradle element 30 is of sufficient depth sothat the bottom of the recess is disposed radially inwardly of thecenter of mass of the cradle element whereby the forces imposed on saidcradle element by the pin 40 will not tend to overturn said cradleelement.

The area of contact between each shroud element 22 and each of itscradle elements 30 is sufficiently large so that during normal operatingspeeds of the rotor the contact pressure of said shroud element againstits cradle elements is sufficient substantially to lock themfrictionally together against relative motion. Accordingly, each shroudelement '22 and its cradle elements 30 could be made as an integral or aone-piece structure. From a fabrication standpoint, however, it issimpler to separately form the shroud and cradle elements asillustrated. Also if, as illustrated, the cradle elements 30 areseparate members which merely have frictional contact with the shroudelements then as the blades stretch in response to centrifugal forcesthereon slight blade vibrations will permit slight shifting of eachcradle element 30 relative to its shroud element 22 to keep its socket42 alined with the associated blade pin 40. The separate fabrication ofthe cradle elements is also important because the cradle elements canthen properly position themselves relative to their respective bladesnothwithstanding the substantial manufacturing tolerances required forthe long thin blade herein contemplated thereby avoiding stresses whichwould otherwise be induced in the blades if the cradle elements were notso properly positioned.

Each shroud element 22 is made of thin lightweight sheet metal, such asaluminum, so that each element is capable of flexing in the regionbetween its cradle elements 60. This flexibility of the shroud insuressubstantially uniform centrifugal loading of the blades 16 through theirpins 40 by their associated shroud elements 22 and cradles '30notwithstanding dimensional differences from one blade to anotherresulting for example, from manufacturing tolerances.

in order to help support the thin sheet metal wall of each shroudelement 22 against the centrifugal forces thereon during rotor rotation,each cradle 30 may, as illustrated, have a plurality of finger-likeextensions 46 and 48. These extensions 46 and 48 in effect divide thearea of the thin sheet metal sections of each shroud element 22 betweenits cradles 30 into a plurality of smaller sections or panels therebyreducing the blending stresses in the shroud resulting from saidcentrifugal forces. It is apparent from FIG. 3 that the general outlineof each cradle 30 with its extensions 46 and 48 is a parallelogram whichis disposed so that the blade chord of the outer end of the bladeassociated with said cradle 30 substantially coincides with a lineparallel to and disposed midway between the long sides of saidparallelogram.

As illustrated in FIG. 1, each shroud element 22 and its supportingblade structure is received within an annular groove 50 formed in theinner wall of the stator housing 52 surrounding the rotor 10. An annularbaflle member 54 supported on the stator housing 52 within the groove 50has its two annular edges 56 disposed so as to closely overlie theoutwardly-turned marginal flanges 29 along the two circumferentiallyextending edges of each shroud element 22 thereby reducing leakage flowaround the outside of the shroud elements 22.

In the embodiment described the pin 40 on each blade bridge element 36and the socket 42 on the associated shroud cradle provide aloose-fitting ball and socket connection therebetween. Obviously,however, in lieu of such a ball and socket connection the relativepositions of each pin 40 and associated socket 42 could be reversed oreven each such bridge element 36 and cradle 30 could both be providedwith a socket with a double ended link being received in said socketsfor transmitting the centrifugal loads. 7

While we have described our invention in detail in its present preferredembodiment it will be obvious to those skilled in the art afterunderstanding our invention that various changes and modifications maybe made therein without departing from the spirit or scope thereof. Weaim in the appended claims to cover all such modifications.

We claim as our invention:

1. A bladed rotor structure for compressors, turbines or like fluidapparatus; said rotor structure comprising a rotor member; a pluralityof circumferentially-spaced blades secured to said rotor member andextending radially outwardly therefrom for co-action with the apparatusfluid; and an annular shroud comprising a plurality of arcuate shroudelements disposed in end-to-end relation across and carried by the outerends of said blades to form the outer boundary of the flow path for saidfluid, each of said blades having a portion secured thereto with aradially-inwardly facing surface and each said shroud element havingportions with radially-outwardly facing surfaces engageable with theinwardly facing blade portion surfaces of a group of blades to restrainsaid shroud element against radially outward movement in response tocentrifugal forces thereon during rotor member rotation and to connectthe outer ends of said group of blades together to damp blade vibrationsof the type causing lateral vibratory motion of the blade outer ends,said radially inwardly facing surface on each blade portion and theradially outwardly surface on the shroud element engaged thereby havinga loose-fitting ball and socket configuration such that said surfacesare arranged for relative frictional movement in response to torsionalvibrations of a blade to damp said torsional vibrations and in responseto bending vibrations of a blade which produce angular motion betweenthe blade outer end and the associated shroud element to damp suchbending vibrations.

2. A rotor structure as recited in claim 1 in which the axis of saidball and socket configuration of said blade portion and shroud portionengaging surfaces being adjacent and substantially parallel to thelongitudinal axis of the blade.

3. A bladed rotor structure for compressors, turbines or like fluidapparatus; said rotor structure comprising a rotor member; a pluralityof circumferentially-spaced blades secured to said rotor member andextending radially outwardly therefrom for co-action with the apparatusfluid; and an annular shroud comprising a plurality of arcuate shroudelements disposed in end-to-end relation across and carried by the outerends of said blades to form the outer boundary of the flov path for saidfluid, each of said blades having a portion secured thereto with aradially-inwardly facing surface and each said shroud element havingportions with radially-outwardly facing surfaces engageable with theinwardly facing blade portion surfaces of a group of blades to restrainsaid shroud element against radially outward movement in response tocentrifugal forces thereon during rotor member rotation and to connectthe outer ends of said group of blades together to damp blade vibrationsof the type causing lateral vibratory motion of the blade outer ends.said facing surfaces being arranged for relative frictional movement inresponse to torsional vibrations of a blade to damp said torsionalvibrations and in response to bending vibrations of a blade whichproduce angular motion between the blade outer end and the associatedshroud element to damp such bending vibrations, each said shroud elementbeing flexible and having a plurality of circumferentially-spaced andrelatively-rigid portions, there being one such rigid shroud portion foreach blade with each such rigid portion having the radially-outwardlyfacing shroud surface which is engaged by the aforementionedradially-inwardly-facing blade portion surface.

4. A rotor construction as recited in claim 3 in which each blade has apair of tenons extending radially outwardly from its tip end andprojecting through openings in its shroud element and in its associatedshroud rigid portion and each blade has a member connected across theouter ends of its said pair of tenons with said member having aradially-inwardly projecting portion disposed between its associatedpair of tenons and with the end surface of said portion being theaforementioned radiallyinwardly-facing blade portion surface.

5. A rotor construction as recited in claim 4 in which each saidinwardly projecting blade portion has a rounded end and each said shroudrigid portion has a recess with a rounded bottom surface engaged by saidrounded end of its associated blade, the radius of the bottom surface ofeach said recess being larger than that of the rounded end engagedthereby.

6. A bladed rotor structure for compressors, turbines or like fluidapparatus; said rotor structure comprising a rotor member; a pluralityof circumferentially-spaced blades secured to said rotor member andextending radially outwardly therefrom for co-action with the apparatusfluid; and an annular shroud comprising a plurality of arcuate shroudelements disposed in end-to-end relation across and carried by the outerends of said blades to form the outer boundary of the flow path for saidfluid, each of said blades having a portion secured thereto with aradially-inwardly facing surface and each said shroud element havingportions with radially-outwardly facing surfaces engageable with theinwardly facing blade portion surfaces of a group of blades to restrainsaid shroud element against radially outward movement in response tocentrifugal forces thereon during rotor member rotation and to connectthe outer ends of said group of blades together to damp blade vibrationsof the type causing lateral vibratory motion of the blade outer ends,said facing surfaces being arranged for relative frictional movement inresponse to torsional vibrations of a blade to damp said torsionalvibrations and in response to bending vibrations of a blade whichproduce angular motion between the blade outer end and the associatedshroud element to damp such bending vibrations, each said shroud elementhaving a flexible sheet metal construction and having a plurality ofindividual relatively-rigid members disposed on its radially-outer sidein frictional contact therewith and circumferentially-spaced therealong,there being one such rigid member for each blade with each such rigidmember having said aforementioned radially-outwardly-facing shroudsurface which is engaged by an aforementioned radially-inwardly-facingblade portion surface.

7. A rotor structure as recited in claim 6 in which each blade has apair of chordwise-spaced tenons extending radially outwardly from itstip end and projecting through a pair of openings in its shroud elementand in its associated rigid member and each blade has a member connectedacross the outer ends of its said pair of tenons with said member havinga radially-inwarldy projecting pin disposed between its associated pairof tenons and with the end surface of said pin being the aforementionedradially-inwardly-facing blade portion surface.

8. A rotor structure as recited in claim 7 in which each said blade pinhas a rounded end and each said rigid member has a recess with a roundedbottom surface engaged by the rounded end of its associated blade pin,the radius of the bottom surface of each said recess being larger thanthat of the rounded pin end engaged thereby.

9. A rotor construction as recited in claim 8 in which the bottom of therecess in each said rigid member is disposed radially inwardly of thecenter of mass of said rigid member.

10. A bladed rotor structure for compressors, turbines, or like fluidapparatus; said rotor structure comprising a rotor member; a pluralityof circumferentially-spaced blades secured to said rotor member andextending radially outwardly therefrom for co-action With the apparatusfluid; and an annular shroud comprising a plurality of arcuate shroudelements disposed in end-to-end relation across and carried by the outerends of said blades to form the outer boundary of the flow path for saidfluid, each of said blades having a pair of tenon portions extendingradially outwardly through openings in its associated shroud element andalso having a member connected across the outer ends of said pair oftenons and strain said shroud element against radially outward movementin response to centrifugal forces thereon during rotor member rotationand to connect the outer ends of said group of blades together to dampblade vibrations of the type causing lateral vibratory motion of theblade outer ends, said facing surfaces being arranged for relative frictional movement in response to torsional vibrations of a blade to dampsaid torsional vibrations and in response to bending vibrations of ablade which produce angular motion between the blade outer end and theassociated shroud element to damp such bending vibrations.

References Cited in the file of this patent UNITED STATES PATENTS1,371,328 Schneider Mar. 15, 1921 1,378,464 Junggren May 17, 19211,639,247 Zoelly et al Aug. 16, 1927 1,998,951 Downer Apr. 23, 19352,315,655 Redding Apr. 6, 1943 2,658,719 Johanson Nov. 10, 19532,963,272 Welsh Dec. 6, 1960 2,970,808 Coppa Feb. 7, 1961 FOREIGNPATENTS 375,169 Great Britain June 23, 1932 377,101 Great Britain July21, 1932

